The present invention relates to a rotating electrical machine, and more particularly, to a rotating electrical machine suitable for an arrangement, which is adopted in, for example, turbine-driven generators, etc. and of which a stator core is formed by laminating magnetic steel sheets.
An explanation will be given to a conventional example of a stator core of a large-sized rotating electrical machine.
As shown in FIGS. 11A and 11B, a stator core of a turbine-driven generator is formed into a cylindrical configuration by punching a steel sheet into sector-shaped split pieces from a steel strip and arranging the split pieces in plural in a circumferential direction to define a circular configuration to laminate the same in plural in an axial direction. FIG. 8 shows an example of the split piece. The split piece comprises teeth 1 with slots 3 therebetween, into which a winding (not shown) is inserted, and a core back 2 defining an outer periphery of the teeth 1.
FIGS. 9 and 10 show a structure of a stator core of a conventional rotating electrical machine. FIG. 9 is a cross sectional view showing the stator core as viewed in a circumferential direction, a half thereof a diametrical direction and a portion close to an axial end thereof. FIG. 10 is a view showing the stator core as viewed from an inside diameter side in a diametrical direction corresponding to a direction indicated by “A” in FIG. 9. As shown in FIG. 10, cooling ducts 5 are formed by interposing rectangular-shaped or I-shaped duct spacing pieces 8 into the stator core 4 to provide for clearances, and a cooling medium is caused to flow diametrically of the cooling ducts 5 to cool the core and an armature winding 6.
FIG. 12 shows a constructional example of a conventional rotating electrical machine. The example comprises a rotating electrical machine of a type having a heat exchanger and causing a cooling medium to circulate therein for cooling.
Usually, a rotating electrical machine comprises a rotor 7 formed by winding a field winding round a rotor core, and a stator arranged in opposition to the rotor 7 with a predetermined spacing (called an air gap) therebetween and formed by winding an armature winding (stator winding) 6 round the stator core 4.
With the rotating electrical machine thus formed, a part of a cooling medium which is increased in pressure by a fan 23 flows from an axial end toward a center along the rotor 7 and toward an air gap which is a diametrical gap between the rotor 7 and the stator to cool the stator core 4 and the armature winding 6 in an exhaust section 26, which extends radially outwardly through the stator core 4, to be subjected to heat removal in a heat exchanger 29 to get again to the fan 23 through a cooling medium flowing path 25.
The remaining cooling medium flows radially outwardly from the fan 23 to cool the stator core 4 and the armature winding 6 in an intake section 27, which extends radially inwardly through the stator core 4, via the cooling medium flowing path 25 to get to the air gap to cool the stator core 4 and the armature winding 6 to join a cooling medium, which flows into the exhaust section 26, in the air gap.
When the rotating electrical machine operates, a winding, through which an electric current passes, constitutes a main heat generating part. In a large-sized rotating electrical machine, with a view to cool such generation of heat, cooling ducts are formed between laminated steel sheets in the manner described above and a cooling medium is caused to flow radially to cool a winding and a stator core.
At this time, in a rotating electrical machine, a cooling effect at neighborhood of the axial center is decreased since the neighborhood of the axial center is positioned most downstream in a cooling medium flowing path and therefore a cooling medium becomes high in temperature. Therefore, a flowing, cooling medium is in some cases increased in quantity by increasing the number of cooling ducts per unit axial length in the vicinity of the axial center.
On the other hand, in a stator core, an alternating magnetic flux acts, so that a loss is generated and heat is generated. In particular, with a rotating electrical machine, which is used in turbine-driven generators to include a rotor having two or four magnetic poles, a path, through which magnetic fluxes pass, in teeth of a core is small in volume to lead to an increase in magnetic flux density, thus causing an increase in heat generation density. Besides, a core which is a magnetic material is decreased in volume in a location, in which the number of cooling ducts per unit axial length is increased as described above, as compared with the remaining locations when locally observed, so that magnetic flux density of the core is increased and a loss is increased. That is, by increasing ducts in number for the purpose of cooling a winding, there is a possibility that loss, that is, heat generation is increased in the core.
With a rotating electrical machine formed with a ventilating circuit, in which a plurality of ventilating sections permitting a cooling medium to flow radially outwardly and radially inwardly of a cooling duct, a cooling medium is increased in temperature in a ventilating section or sections positioned downstream of the ventilating circuit.
On the other hand, in teeth positioned at an axial end of a stator, an eddy current is generated in an axial cross section of the teeth by a magnetic flux incident in an axial direction from a rotor. While the magnitude of a loss caused by the eddy current is dependent upon the density of an incident magnetic flux and the cross sectional area of the tooth, a location of heat generation is local and tip ends of that teeth, on which a loss is caused, is made high in temperature in some cases. In particular, with a machine involving a large electric loading, a magnetic flux leaking from a coil end is increased in quantity to lead to an increase in loss for reasons of a large electric current and a large coil end length, and therefore, a measure of reduction in loss and enhanced cooling such as the provision of a tapered portion at an end of a stator core and the provision of a slit on an axial cross section of teeth is taken as proposed in JP-A-2006-74880.
In relation to the matter described above, as a measure, which copes with generation of a loss on teeth, in particular, at an end of a stator core, U.S. Pat. No. 7,057,324 shows an example, in which a direction of easy magnetization of a steel sheet and a direction of magnetic flux in teeth agree with each other at an end of a stator core. Since a direction of magnetic flux in teeth and a direction of magnetic flux in a core back are perpendicular to each other, however, a magnetic flux is hard to pass through the core back, so that there is a possibility of an increase in loss.
As an example of reduction in generation of heat on a core, JP-A-2000-50539 shows an example, in which grain oriented steel sheets and non-oriented steel sheets are alternately laminated, and JP-A-61-62334 shows an example, in which amorphous metallic sheets are laminated at an end of a stator core. In all the examples, an attention is paid to easiness, with which a magnetic flux passes through laminated steel sheets, and to iron loss, and steel sheets of plural kinds are laminated to achieve reduction in heat generation.
However, a magnetic material is nonlinear in magnetization characteristic and easiness, with which a magnetic flux passes therethrough, that is, magnetic permeability is changed according to the density of a magnetic flux, which acts. In recent years, many rotating electrical machines are designed to be increased in power density and made small in size and the density of a magnetic flux acting on a stator core is set to a high density of a magnetic flux close to saturation magnetization. In the case where a magnetic flux density close to saturation magnetization is acted, a magnetic resistance increases since a magnetic material is equivalent in magnetic permeability to an air, so that there is also a possibility that a magnetic flux makes a circuit in an axial direction in the same manner at the end as described above and a magnetic flux in a direction of lamination increases to lead to an increase in loss.